This invention relates to single-pass inkjet printing.
In typical inkjet printing, a print head delivers ink in drops from orifices to pixel positions in a grid of rows and columns of closely spaced pixel positions.
Often the orifices are arranged in rows and columns. Because the rows and columns in the head do not typically span the full number of rows or the full number of columns in the pixel position grid, the head must be scanned across the substrate (e.g., paper) on which the image is to be printed.
To print a full page, the print head is scanned across the paper in a head scanning direction, the paper is moved lengthwise to reposition it, and the head is scanned again at a new position. The line of pixel positions along which an orifice prints during a scan is called a print line.
In a simple scheme suitable for low resolution printing, during a single scan of the print head adjacent orifices of the head print along a stripe of print lines that represent adjacent rows of the pixel grid. After the stripe of lines is printed, the paper is advanced beyond the stripe and the next stripe of lines is printed in the next scan.
High-resolution printing provides hundreds of rows and columns per inch in the pixel grid. Print heads typically cannot be fabricated with a single line of orifices spaced tightly enough to match the needed printing resolution.
To achieve high resolution scanned printing, orifices in different rows of the print head can be offset or inclined, print head scans can be overlapped, and orifices can be selectively activated during successive print head scans.
In the systems described so far, the head moves relative to the paper in two dimensions (scanning motion along the width of the paper and paper motion along its length between scans).
Inkjet heads can be made as wide as an area to be printed to allow so-called single-pass scanning. In single-pass scanning, the head is held in a fixed position while the paper is moved along its length in an intended printing direction. All print lines along the length of the paper can be printed in one pass.
Single-pass heads may be assembled from linear arrays of orifices. Each of the linear arrays is shorter than the full width of the area to be printed and the arrays are offset to span the full printing width. When the orifice density in each array is smaller than the needed print resolution, successive arrays may be staggered by small amounts in the direction of their lengths to increase the effective orifice density along the width of the paper. By making the print head wide enough to span the entire breadth of the substrate, the need for multiple back and forth passes can be eliminated. The substrate may simply be moved along its length past the print head in a single pass. Single-pass printing is faster and mechanically simpler than multiple-pass printing.
Theoretically, a single integral print head could have a single row of orifices as long as the substrate is wide. Practically, however, that is not possible for at least two reasons.
One reason is that for higher resolution printing (e.g., 600 dpi), the spacing of the orifices would be so small as to be mechanically unfeasible to fabricate in a single row, at least with current technology. The second reason is that the manufacturing yield of orifice plates goes down rapidly with increases in the number of orifices in the plate. This occurs because there is a not insignificant chance that any given orifice will be defective in manufacture or will become defective in use. For a print head that must span a substrate width of, say, 10 inches, at a resolution of 600 dots per inch, the yield would be intolerably low if all of the orifices had to be in a single orifice plate.
Paper that is moved along its length during printing has a tendency (called web weave) to move back and forth in a direction perpendicular to the intended printing direction, which can degrade the quality of printing. In addition, when a broad area that includes several adjacent lines is to be printed, variations in the lateral spread rates of the edges of the lines and groups of already merged lines that will form the area may yield unintentionally non-printed areas.
The invention provides effective tradeoffs between a pattern for staggering parallel print arrays in a swath module of the print head that provides optimal latitude relative to web weave; and one that provides optimal line spreading behavior.
In general, in one aspect of the invention, a print head has an array of ink orifices arranged to selectively deposit drops of ink along parallel print lines on the medium while the medium and the print head undergo relative motion in a printing direction parallel to the print lines, the printing being completed in a single pass of the print head relative the medium. The orifices in the array are arranged in a pattern such that adjacent parallel print lines on the medium are served by orifices that have different positions in the array along the direction of the print lines. The different positions of the orifices that serve any pair of adjacent parallel lines are separated by no less than a first predetermined distance along the direction of the print lines.
Implementations of the invention may include one or more of the following features. The different positions may also be separated by no more than a second predetermined distance along the direction of the print lines. The ratio of the largest distance to the smallest distance separating any pair of adjacent orifices may be in the range 1:1 to 2:1, e.g., 1.4:1. The first and second predetermined distances may be chosen to yield a maximum overlap of adjacent line printing. The print head may include swath modules each of which includes array modules that are staggered to achieve the pattern. The staggering of orifices may be in a saw-tooth pattern. The pattern of staggering of one of the swath modules may be congruent to the pattern of staggering of another of the swath modules. The medium may be non-absorbent.
In general, in another aspect, the invention features a method of printing on a medium in which pairs of print locations that are on adjacent print lines and that are on an imaginary line normal to the print direction are caused to be printed at times that are separated by a delay period of at least a predetermined duration. In implementations of the invention, the delay period is also at most of a second predetermined duration.
In general, in another aspect, the orifices in the array are arranged in a pattern in which each of the orifices is either upstream or downstream of both of the neighboring orifices along the printing direction.
In general, in another aspect, the invention features a swath module for use with other modules in a single-pass print head.
Among the advantages of the invention are one or more of the following.
The effects of web weave and line spreading are traded off in a useful way while reducing the cost of orifice plate manufacture. The invention is especially applicable to printing on a nonabsorbent medium and to printing that involves merging of print lines while the ink remains liquid.
Other advantages and features will become apparent from the following description and from the claims.